An ink jet recording system is a recording system in which ink liquid droplets are jetted directly on a recording member from very fine nozzles and made to adhere to the recording member to obtain characters and images. This system has not only the advantage that the device to be used has a low noise and good operability but also has the advantage that coloration is easy and plain paper can be used as recording members and is therefore widely used. As ink used for an ink jet printer, pigment or hydrophobic dye type ink has been developed or used in recent years to improve water fastness and light fastness.
However, the pigment or hydrophobic dye ink scarcely remains on the surface of plain paper, posing a problem as to reduced optical density. Also, there is the problem that if the content of a pigment or a hydrophobic dye is increased in an ink formulation, the viscosity of ink is increased, giving rise to the problem concerning a deterioration in ink jetting ability and preserving stability.
In pigment ink, it is proposed, for example, that a polymer containing a polyethylene glycol (meth)acrylate monomer and α,β-ethylenic unsaturated carboxylic acid is added in order to improve printing quality and optical density (JP-A 6-306317 and the equivalent U.S. Pat. No. 6,011,098) and that a polymer dispersant containing a polyethylene glycol(meth)acrylate monomer or a poly(trimethylene glycol)(meth)acrylate monomer (JP-A 2000-144031) and the like is added to improve dispersibility and preserving stability. It is also proposed that macromer is compounded to improve high light fastness and high optical density (WO-A 00/39226 and the equivalent EP-A 1059341). Moreover, for example, aqueous ink containing a water dispersion of polymer particles of a water-insoluble vinyl polymer of a monomer using a combination of an ethylene oxide and a propylene oxide which ink has excellent water fastness, rub fastness and dispersion stability is proposed (JP-A 2004-2662 and the equivalent EP-A 1323789).